The present invention relates to a method of launching of large-size submarine pipelines, starting from the mainland, where they are previously assembled from shorter sections.
More particularly, the present invention relates to the laying of pipelines for crossing stretches of fresh water or stretches of sea of small size, of water streams, or for water intakes for industrial works, the assemblage of which is carried out on the mainland, with the assembled pipeline then being launched by being pulled from its offshore end, by means of an anchored pontoon, or by a winch with an offshore return.
According to the launch technology, the pipeline section assembled on the mainland must be launched while being equipped with suitable weights, so that the structure being launched has an overall specific gravity slightly higher than the water in which it is being launched.
With a specific gravity slightly higher than that of the medium in which the pipeline is being launched, the pipeline section is permitted to maintain, while it is being laid, a residual water weight which keeps it in correspondence to the profile of the water bottom and in an assigned position, by counteracting the thrust applied to it by possible streams, current motion or other environmental stresses. At the same time, the pipeline being launched should not show too high a frictional resistance to the launching operation.
With the equipment for pulling the pipeline being the same, the preassembled pipeline sections can be longer, the lower the "specific" resistance offered by the pipeline.
According to the technique of the prior art, various modalities are presently applied for increasing the weight of the pipeline. Such modalities are now described briefly.
The most widespread launching method consists in welding onto the offshore end of the pipeline which is being launched, a blind component, of either a conical or rounded shape, provided with means for anchoring the pulling cables, which allows the pipeline to be tightly sealed; maintaining the pipeline in an empty state, and increasing its weight by applying onto its outer surface a coating of cement mortar of various compositions which is provided with reinforcing elements ("concrete spraying" process).
The increase in weight obtained by means of the concrete spraying method is very inaccurate inasmuch as practically obtaining, at the end of the operation, the theoretical thickness and hence the theoretical weight of the material being applied onto the pipeline, in particularly, in case of large-diameter pipelines, is difficult. Furthermore, the concrete spraying process results are expensive, both because of the need for large amounts of the required materials and due to the long processing times required for their application, hardening, and their necessary ageing, before it is possible for the pipeline to slid on the bottom.
Another draught method for increasing the weight of the pipeline consists in applying, to the mouth of the end of the pipeline being launched, a blind component, as in the former method, and in then flooding the pipeline. The specific weight of the pipeline is maintained at the desired value by applying, along the length of the pipeline, a plurality of floating bodies, which support it.
According to a more common variant of such a method, the floating bodies are applied externally to the pipeline, at discrete intervals in the axial direction, by means of suitable fastening means (brackets, ropes, chains, etc.). At the end of the launch, the floating bodies and their fastening means are removed and recovered. Such a variant has drawbacks derived from the fact that, especially during the initial portion of their run together with the pipeline, the buoys as well as the fastening means undergo creeping, and damage.
In addition to the costs derived from the possible replacement and repair of such buoys and fastening means, such damage requiters the launching operations to be stopped to the purpose of repairing the damaged buoy, thus lengthening the entire installation process.
In order to obviate such drawbacks, according to an improved variant having a such a method, floating bodies of long shape were placed inside the pipeline.
Such a feature overcomes the drawbacks characterizing the former variant, but causes notable complications, as regards the proper positioning of the floating bodies inside the pipeline, and the procedure for recovering them, at the end of the pipeline launch.
During the launching operation, the buoys used to attain the desired residual weight of the water-filled pipeline, applied both inside the pipeline and externally to it, are dragged to increasing depths.
This fact leads to a considerable drawback for the buoys made of such pneumatic bodies as balloons or cylinders filled with compressed air, which are the buoys endowed with the best handling and economy characteristics.
Such pneumatic bodies, placed at an increasing depth, shrink due to an increase in the outer pressure. To restore their value of positive, upward buoyancy, such bodies have to be inflated, as the launch brings them to increasing depths, in order to bring them back to their initial value.
As an alternative, it is also possible to change the length of the connection between the pipeline being launched and the outer buoys so be to keep these buoys at a substantially constant depth.
Both of these solutions are complex.
The adoption of rigid buoys, able to withstand the outer pressure, requires the use of rather heavy, poorly handeable metal structures.
Both of the procedures of application of the buoys during the launch, and the procedure for the recovery of the buoys, at the end of the launch, are complex.
The techniques exposed up to the present suffer from the drawback that the increase in weight conferred to the pipeline cannot be easily adjusted or modified during the laying procedure, should, e.g., the unevennesses of the bottom or changes in the ambient conditions require the residual weight of the whole pipeline, or portions thereof, to be changed.
The laying process according to the present invention allows the weight of a pre-assembled pipeline to be increased with precision, to a desired weight value for a pipeline being launched from the mainland, and said weight can be varied, both over time and along the various sections of the pipeline. Also the launch can be carried out easily, quickly and economically.
Such a proces of the present invention shall be disclosed, by referring to the FIGURE, in the specific application to the launch of a large-size pipeline destined as water intake installations for industrial plants.
However, the following disclosure should not be considered as being limitative of the scope of the present invention, in that the present process can be used also for large-size pipelines for different uses.
Such pipelines are characterized indeed by very large flowrates, and thus large diameters--up to more than 2,500 mm--as well as by low operating pressure drops, and, consequently, by very limited wall thicknesses within the range of from 15 to 30 mm, which render such pipelines very delicate during the launch operations, should they undergo stresses not uniformly distributed lengthwise to the pipeline.
According to the coastal site the water intake units for industrial plants are being installed at, these units have a length which is usually within the range of up to 3 km, and are layed at such a depth as to guarantee the intake of clean water, at a substantially constant temperature during the medium term and are, not influenced by day to day temperature changes. According to the site, the depth, in the most frequent cases, is within the range of from 10 to 15 meters.
By starting from the intake end, equipped with various grids and devices, to prevent foreign bodies of discrete dimensions to enter the pipeline, the pipeline extends up to the mainland, wherein the industrial works are installed. In many instances, in the intake unit, in a sea environment, means are installed for the supply, the metering and the distribution of additives to prevent, or, at least, limit, the growth of colonies of marine flora and/or fauna--seaweeds, shellfish and the like--which may occlude the pipeline, within a short period of time, rendering it useless.
The pipelines of the intake installations are provided, at discrete lengths, with large manholes, which allow skin-divers, provided with aqualungs, to enter the inside of the pipeline, to periodically inspect the pipeline and eventually service it.
The length of such sections, between the manholes, is generally within the range of from 50 to 120 m, and the manhole has an inner diameter of from 1200 to 1800 mm, which allows the skin-divers and their equipment to easily enter the pipelines.